Sphalerite geobarometry in metamorphic terranes: an appraisal with implications for metamorphic pressure in the Otago Schist

Abstract

Abstract Sphalerite geobarometry has long been known to give poor results when applied to regional metamorphic terranes. Application of the sphalerite geobarometer to three low‐to medium‐grade sulphide deposits—the Moke Creek and Waitahuna deposits, Otago, and the Goose Cove deposit, Newfoundland—yields pressures up to 9 kbar, which appear to be too high when compared with other geological data. Textural and mineralogical relationships suggest that the Goose Cove and, possibly, the Moke Creek deposits lacked the required equilibrium assemblage (pyrite + hexagonal pyrrhotite + sphalerite) during peak metamorphic conditions, rendering the geobarometer inapplicable. In addition, all three deposits show evidence of re‐equilibration at T < 300°C, which has resulted in decreased FeS contents and high apparent pressures. Analyses of sphalerites from very low‐grade metachert from South Georgia Island, which contains the assemblage sphalerite + pyrite + monoclinic pyrrhotite + chalcopyrite, confirm that low‐temperature equilibration of this assemblage results in approximately 10–11 mol. % FeS in sphalerite. Comparison of these results with published descriptions of other deposits suggests that lack of the appropriate assemblage and retrograde re‐equilibration of sphalerite probably account for most anomalously high‐pressure estimates. Erratic compositions of sphalerites containing chalcopyrite inclusions may result from replacement of high‐temperature intermediate solid‐solution by chalcopyrite during cooling. Strain may enhance retrograde re‐equilibration of sphalerite by grain‐size reduction or dislocation‐assisted diffusion and/or nucleation. Re‐evaluation of the data from Moke Creek suggests that the sulphides experienced pervasive greenschist facies re‐equilibration at pressures of about 4.5 kbar, with late stage mobilization at about 2.8 kbar, and thus sphalerite compositions are not likely to reflect blueschist facies conditions. Pressure estimates based on sphalerite geobarometry should take into account at what stage in the history of a metamorphic terrane the sphalerite composition equilibrated.